The present invention also relates to a shoe provided with said sole.
It is known that the comfort of a shoe is linked not only to correct anatomical fit but also to correct outward transmission of the water vapor formed inside the shoe due to sweating, in order to avoid the “damp foot” phenomenon.
This permeation of the water vapor, however, must not compromise the waterproofness of the shoe, and therefore solutions have been studied which entrust vapor permeation to the upper or to the sole.
As regards the upper, there are shoes with an upper made of vapor-permeable and waterproof material.
However, this type of shoe does not effectively solve the problem of vapor permeation: the “damp foot” problem arises from the fact that during sweating the sweat evaporates from the foot and cools it. If evaporation is prevented, the sweat condenses and leaves the foot damp.
Most of the sweat of the foot is originated at the interface between the sole of the foot and the sole of the shoe, and it is evident that the sweat formed here is unable to evaporate, thus condensing on the plantar element on which the foot rests. Only a minimal part of the sweat evaporates through the upper, even if said upper is reduced to a minimum.
Solutions to the problem are provided by vapor-permeable and waterproof soles, which accordingly allow permeation of the sweat generated at the sole of the foot.
One of these solutions is disclosed in U.S. Pat. No. 5,044,096 and EP-0382 904 by the same Applicant and consists in dividing the sole into two layers with through holes and in interposing a membrane that is impermeable to water and vapor-permeable and is joined perimetrically and hermetically to the two layers.
Variations of this solution are disclosed in subsequent patents, all of which are in any case centered on dividing the sole into two layers, with the interposition of a waterproof and vapor-permeable membrane.
In view of the fact that most of the sweat of the foot is generated at the interface between the sole of the foot and the sole of the-shoe, the “damp foot” problem is considerable even in open shoes, such as sandals, sabots, mules for seaside use, et cetera.
These kinds of open shoe are generally constituted by an upper that only partially wraps around the foot insertion region and by a multilayer sole.
With reference from the top downwardly, such multilayer sole comprises a first layer, which makes direct contact with the foot and is generally made of leather or synthetic material.
If such layer is made of non-vapor-permeable material (such as for example mules for seaside use), sweat condenses immediately, leaving an almost instantaneous feeling of dampness at the foot.
If the layer is made of a vapor-permeable material that has some absorbent power, the foot remains dry until such layer becomes impregnated with sweat.
Below the first layer there is a second layer, which is generally contoured anatomically and is generally made of an expanded or compact material, such as polyurethane, cork, wood, expanded rubbers, et cetera.
Finally, there is a tread, which makes contact with the ground and can be made of synthetic material (rubber, polyurethane), natural material (leather), or mixed material.
The various layers are coupled by means of adhesives or, as an alternative, by injection-molding the various layers of polymeric material over the others.
Below the second layer, if it is made of soft or low-consistency material, there is a supporting layer, so as to provide a rigidity and hardness that allow to glue the flaps of the upper between the tread and said supporting layer.
For open shoes that must have a certain comfort degree, it is important that the supporting plantar element (or insole) be sufficiently soft and anatomically contoured.
This leads to the need to use, for the second layer (and optionally also for the third layer), soft materials, such as “sponges” or the like, in order to accommodate the foot and surround its sole.
However, this surrounding prevents correct circulation of the air between the skin of the foot and the plantar element on which the foot rests, leading to almost immediate condensation, which cannot be dissipated through the sole.
Furthermore, the foot transfers heat to the surrounding material, which retains it and produces a “padding” effect that further overheats the foot.
If a rigid plantar element were used, the foot would have fewer points of contact with said plantar element, allowing better ventilation and optimum cooling of the foot.
Therefore, as the rigidity of the plantar element increases (and therefore the comfort of the shoe decreases), the freshness of the foot increases. Vice versa, the lower the rigidity and hardness of the plantar element (and therefore the greater the comfort degree of the shoe), the stronger the “sweaty foot” effect.
Moreover, as already mentioned, lack of vapor permeation of the layers below the foot leads to almost immediate condensation on the sole of the foot. This lack of permeation is due both to the type of materials used (particularly for the tread) and to any films of adhesive that join the various layers.
The causes of the “sweaty foot” effect in open shoes provided with an anatomically contoured soft plantar element can therefore be summarized in three aspects: the surrounding of the foot by the anatomically contoured layer, which prevents correct recirculation of air between the sole and the plantar element; the “padding” effect caused by said surrounding; and the lack of permeation of the sweat in the direction of the tread.